1. Field of Invention
This invention relates to an apparatus for sensing chemical properties of sprays used in flue gas desulfurization systems. More specifically, this invention relates a novel apparatus used to house a conventional sensor for sensing the pH of a spray that is used to absorb sulfur dioxide from flue gas produced by coal combustion.
2. Description of the Related Art
This invention relates to process control of systems which remove sulfur dioxide (SO.sub.2) from flue gas produced by coal combustion; more specifically, it relates to absorbers in which SO.sub.2 laden gas passes through a spray of finely divided limestone or lime in aqueous solution. This spray is introduced into absorbers by sprayers that convert fluid in the form of a slurry into a spray. SO.sub.2 is present in waste gases generated from combustion of fossil fuels, for example, in electrical power generation. SO.sub.2 is also generated in waste gases from metal ore processing. The two types of waste gases just mentioned are commonly referred to as "flue gases." Since SO.sub.2 has long been recognized as an atmospheric pollutant, desulfurization systems are in widespread use. Even small improvements in efficiency of SO.sub.2 absorbers can be highly beneficial to the public due to the large numbers of absorbers in use and due to the large volumes of flue gases processed by them.
Some desulfurization systems use bases (alkalines) to treat SO.sub.2 containing gas. Finely divided limestone or lime is commonly used as the alkaline material. An example of such a desulfurization system is disclosed by U.S. Pat. No. 4,388,283 entitled "SO.sub.2 REMOVAL" which patent is hereby incorporated by reference.
In limestone or lime desulfurization systems, an aqueous slurry of finely divided limestone or lime is pumped through sprayer nozzles in an absorber. The chemically active constituents (i.e., the alkaline reagents) of these sprays are calcium carbonate (CaCO.sub.3) for limestone and calcium hydroxide (CaOH) for lime. SO.sub.2 laden gas is forced into the absorber and then forced through the spray. When SO.sub.2 passes through the spray, a chemical reaction occurs in which SO.sub.2 combines with the alkaline reagents to form insoluble calcium sulfite (CaSO.sub.3) and calcium sulfate (CaSO.sub.4), and water, H.sub.2 O. Spray traverses a spray zone in the absorber which spray zone is the location at which SO.sub.2 absorption occurs. After traversing the spray zone the spray collects as a slurry in a reaction tank from where it is recycled to be again sprayed through the spray zone. The pH and other chemical properties of the slurry to be recycled are adjusted, as desired, through addition of water and limestone.
Continuous adjustment and control of the pH of limestone spray is important since effectiveness of SO.sub.2 removal is a function of spray pH. However, spray pH control is problematic since this pH depends upon factors such as natural variations in limestone, the degree of fineness to which limestone is ground, and the weight percent of limestone in the aqueous slurry. Moreover, the optimal pH varies depending upon the chemical composition of both the spray and the gas. Finally, the chemical composition of the gas varies with natural variations in the composition of fossil fuel and metal ore.
A further complication is present since the pH of particular spray droplets changes in response to reaction with flue gas as the spray droplets traverse the spray zone within the absorber. It is desirable that the spray be chemically effective throughout the spray zone rather than only effective as the spray enters the spray zone. Finally, pH must be controlled within lower and upper limits so that the spray does not become to acidic, which would cause corrosion of the absorber system, and so that the spray does not become too basic, which would cause scale formation and clogging of the system, particularly clogging of the system piping with precipitates.
In some absorption systems, the pH control point is the slurry as it is recirculated to the sprayer. That is, pH of the slurry is sensed at a recirculation line and pH is adjusted according to the pH measurement there sensed. Another pH control point is the reaction tank. However, neither of these control points provides for sensing the pH of the spray in the spray zone. Moreover, the varying pH at different locations within the spray zone is not sensed at either of those control points. The reaction tank has additional limitations as a control point since the large volume of slurry contained within it acts to heavily buffer the effects of spray having a different pH from the slurry.
The limitations mentioned in the previous paragraphs inspire a need for a novel apparatus designed to permit sensing of pH at various locations within the spray zone. In order to accurately sense pH, the sensor's electrode, that is, the sensing element of the sensor, should be completely immersed in the spray. This precludes a simple insertion of the sensor into the spray zone since such insertion would provide neither an accurate nor a reliable pH indication.
Measuring pH in the spray zone is difficult to accomplish because the spray zone is a hostile environment for a pH sensor. Spray is emitted in large quantities at high pressure and spray has abrasive constituents. The spray so emitted is not unlike the effect of a sandblasting process. Direct exposure of a sensor to this environment may result in accelerated sensor erosion requiring frequent replacement of costly sensors. The delicate nature of pH sensors also calls for sensor protection from the high slurry forces and gas turbulences that are characteristic of the spray zone.
A novel apparatus is therefore desired which will allow a conventional pH sensor to be immersed in spray at various locations within the spray zone, the key location for pH measurement in SO.sub.2 absorbers, and which will protect the sensor from excessive wear and premature breakage.